Radio Frequency Identification (RFID) is a technology used to identify a target object. An RFID system typically includes an information carrying module known as a tag. A tag may include a microchip attached to an antenna and may be packaged such that it can be applied to the target object. The tag receives and transmits signals to and from a reader, most often packaged in the form of a transceiver. The tag may contain a unique serial number as well as other information, such as a customer account number. Tags may be implemented in many forms. For example, a tag may have a barcode label printed thereon, may be mounted inside a carton or may be embedded within the target object.
RFID tags may be implemented as active, passive or semi-passive devices. RFID tags function in response to coded RF signals received from a base station transceiver. An active tag generally includes its own energy supply, such as a battery, that may serve as a partial or complete power source for the tag's circuitry and antenna. Batteries may be replaceable or sealed units. A passive tag is generally powered by the RFID reader itself and thus generally does not contain a battery. It typically communicates by reflecting an incident RF carrier back to the reader. As radio waves from the reader are encountered by a passive RFID tag, a voltage is induced from the antenna. The tag may draw power from the induced voltage to energize its circuits. The tag then transmits the encoded information stored in the tag's memory by backscattering the carrier of the reader. A semi-passive RFID tag typically uses a battery to operate its internal circuitry, but also relies on backscattering communication.
Reading is the process of retrieving data stored on an RFID tag by propagating radio waves to the tag and converting the waves propagating from the tag to the reader into data. Information is transferred as the reflected signal is modulated by the tag according to a particular programmed information protocol. Protocols for RFID tags may be categorized in terms of tag to reader over the air interfaces. Three common interface classes are Class-0 (read-only), Class-1 (read/write), and UHF Generation-2 (read/write), for example. UHF Generation-2 provides some improvement upon existing Class-0 and Class-1 standards for worldwide operation and improved performance, for example. ISO18000 standard series RFID tags cover both active and passive RFID technologies.
A direct conversion based reader has a receive path which down converts the reflected carrier to DC (direct current). FIG. 1 shows an example of a known receive path 10 which includes a high pass filter 12 which removes the reflected carrier DC component from the signal received from the antenna. RF signals from one or more tags may be received at input 14 from an antenna and are provided to an input downconverter 16 comprising an amplifier 18. A local oscillator 20 may be provided to generate frequencies for mixing with the received RF signal in a mixer 22 to perform the down-conversion.
The filter 12 includes a direct current (DC) blocking capacitor 24 which removes the DC component of the baseband signals. After the blocking capacitor 24 removes the DC component, the signals may be amplified by suitable baseband amplifiers and filters as represented by the component 26 and converted to digital signals by suitable analog to digital converters. RFID receivers or transceivers may use one or more receive paths to handle various classes of tags (e.g., Class-0, Class-1, and UHF) in a given environment. RFID receivers or transceivers may use other types receiver circuitry including superheterodyne receivers such as those utilizing a DC or near DC frequency as a second intermediate frequency.